1. Field of Technology
The present invention generally relates to optical systems. More specifically, the present invention relates to noise reduction in the transmission of optical signals.
2. Description of Background Art
In recent years, optical systems have become increasingly desirable for sensor applications as well as magneto-optical storage systems. In a magneto-optical (MO) storage system, a laser in conjunction with an optical fiber are used with a MO media to attain data storage densities beyond conventional magnetic data storage systems. This type of system relies upon propagating a light signal through an optical fiber and reflecting off of a MO media. Based upon the polarized logic state of the location on the MO media, which the light signal contacts, a Kerr effect slightly alters the polarization of the light signal, thereby enabling the light signal to carry the logic state of the MO media back to a differential detector. The differential detector transforms this polarized light signal into an electrical data signal, which is processed by a computing device, such as a personal computer.
In sensor systems, fiber optic current sensors include a sensing optical fiber, which is wound with an integral number of turns around a current carrying wire and with each point in the sensing fiber having a constant sensitivity to the magnetic field. Current flowing through the wire induces a magnetic field, which through the Faraday effect, rotates the plane of polarization of the light traveling in an optical fiber around the current carrying wire. This rotation of the state of polarization of the light due to the electrical current is measured by either injecting light with a well defined polarization state into the designated sensing region and analyzing the polarization state of the light after it exits this same sensing region or by measuring the velocity change of a circularly polarized light wave traversing the sensing region.
By utilizing a multi-mode laser and propagating a main light signal through a frequency selective polarization-maintaining (PM) optical fiber, slight unavoidable optical misalignment errors result in enhanced laser noise, such as mode partition noise (MPN) being converted into intensity and polarization noise, thus degrading the performance of the optical system. Even though the overall intensity noise of such a multi-mode laser is low, the noise present in each individual mode is very high.
One solution for minimizing MPN is to utilize a single-mode distributed feedback (DFB) laser, which does not generate these multiple modes within the system, thereby avoiding the effects of MPN. However, DFB lasers, which operate in the red spectral range and at high power levels, currently are not readily available on the commercial market. In addition, since multi-mode laser diodes are considerably less expensive compared to DFB lasers, multi-mode lasers are the preferred type of laser source for many optical systems.
What is needed is a system and method that utilizes the multi-mode laser, but minimizes the effects of MPN within the optical system.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing a system and method that minimizes the mode-partition noise (MPN) contribution by depolarizing the noise generating parasitic light signals. In particular, a preferred embodiment of the system includes a multi-mode laser, a leaky beam splitter (LBS), a first half wave plate (HWP1), a second half wave plate (HWP2), a depolarizer, an optical fiber, a first quarter wave plate (QWP1), a second quarter wave plate (QWP2) and a differential detection module.
The multi-mode laser generates the main light signal, which is used as a read signal for carrying the polarization state of the main light signal from a specific location on the MO media or the sensing region to the differential detection module. The laser is modulated on and off at a radio frequency, which is dependent upon the optical path lengths associated with the depolarizer and the optical fiber. The depolarizer and the optical fiber are part of a continuous birefringent conduit for the polarization and propagation of the main light signal between the reflective medium or sensing region and the differential detection module.
The HWP1 and the HWP2 in conjunction with the QWP1 align and polarize the main light signal to ensure that the first and second components of the main light signal propagate along each optical path length of the depolarizer and each axis of the optical fiber By propagating along one optical path length and axis on a forward path and the opposing optical path length and axis on a return path from the reflective medium or sensing region, the two components of the main light signal will have a net zero optical path difference. To minimize MPN contribution, which is caused by the parasitic waves created by misaligned system components, the depolarizer depolarizes these parasitic light signals, so as to destroy coherent interference between the two parasitic light signals, which minimizes the MPN effects on the main light signal.
The LBS, which allows a main polarized mode of light to enter the depolarizer and the optical fiber on the forward path, reflects on the return path part of the main polarized mode and most of the perpendicularly polarized mode of the main light signal toward the differential detection module. The QWP2 reorients the reflected main light signal to ensure that the logic states within the main light signal are more easily detectable by the differential detection-module.